docs: def _to_string(x):
      """Convert to a unicode string.
  
      If x is a byte string, assume it is utf-8 and decode to a Python unicode
      string. You must call this on text strings you get back from libvips.
  
      """
      if x == ffi.NULL:
          x = 'NULL'
      else:
          x = ffi.string(x)
          if isinstance(x, byte_type):
              x = x.decode('utf-8')
  
      return x